Laser anemometry notably allows the determination of the displacement velocity of a gas, for example a flow of air around an aircraft, by exploiting the presence of suspended aerosols. These aerosols, also termed particles or tracers are for example dust particles or droplets. An optical beam is emitted by a laser and backscattered by aerosols suspended in the air, then the measurement of the frequency shift between an optical reference signal and the backscattered signal allows the velocity of the carrier relative to the aerosols, and thus also to the air, to be determined. The frequency shift thus measured is called the Doppler frequency; it is directly proportional to the component, along the optical propagation axis, of the relative velocity vector of the aircraft with respect to the mass of air. Conventionally, this technology may be split into two classes: multi-particle laser anemometry and single-particle laser anemometry.
Multi-particle laser anemometry consists in probing a relatively large volume of gas—containing a large number of particles (for example several million particles)—at a somewhat large distance from the laser source—of the order of 100 meters, for example—so as to obtain a continuous low-power backscattered signal. This technology requires a high emission power. It therefore requires the use of significant hardware resources and may constitute a safety risk for personnel due to the presence of a high-energy laser.
Single-particle laser anemometry is obtained with the continual emission of a laser over a short distance and focused onto a small volume of gas, thus enabling the observation of particles separately and with a high illumination level (so that the signal-to-noise ratio of the backscattered signal is positive, preferably at least equal to several dB). The backscattered signal then takes the form of a series of pulses that appear randomly, the pulses being produced each time a particle passes through the volume illuminated by the laser. In reality, when the airspeed of an aircraft is measured by laser anemometry, it is not the velocity of the gas that is measured but the velocity of the aircraft relative to this gas (the air in this case), the particles suspended in the air being virtually stationary. It is therefore the displacement of the measurement volume (due to the velocity of the aircraft) which creates a series of pulses. The expression “gas displacement velocity” should be understood to mean “relative velocity” with respect to the light beam.
The single-particle velocity measurement method requires little power and is easily applied to a homogenous medium, in other words to a gas containing particles of similar sizes. However, when it is wished to use this method to measure the gas displacement velocity of a heterogeneous medium, for example to determine the airspeed of an aircraft, the velocity measurements obtained are sometimes inconsistent, notably because of the presence of large particles (pollen grains, ice micro-crystals, rain, etc.) which do not move at the same velocity as the flow of air in which they are suspended, in particular because of the disturbance caused by the aircraft on the nearby mass of air. Since the gas displacement velocity is estimated, for example, by calculating the average velocity of many particles, taking account of particle velocities not moving at the same velocity as the gas lowers said estimation.
Furthermore, the choice of the focal volume, which is to say the volume in which the passage of a particle of the expected size produces a backscattered pulse of sufficiently high amplitude to be useful, is directly related to the distance between the laser source and the focal point of this laser. The choice of said volume—and therefore of the focal distance—results from a compromise between the desire to have sufficient particles to observe and the desire to observe these particles separately from one another. Applied to anemometry on an aircraft, this compromise generally results in a choice of focal distance of the order of several meters, thus being in the zone aerodynamically disturbed by the aircraft. The aerodynamically disturbed flow close to the fuselage leads to the presence of a velocity gradient along the optical propagation axis of the laser. In other words the particles suspended in the air are not moving at the same velocity depending on their position on this optical propagation axis. Thus, a high selectivity along said axis is desirable in order to limit the effect of the velocity gradient.